1. Field of the Invention
The present invention relates generally to a safety device, and in particular to a push rod of a safety device of a switch, wherein the push rod ensures that a conductive plate is deformed to cut off the circuit.
2. The Prior Arts
A conventional safety device for a switch is used to electrically connect the two terminals when the circuit is in “ON” status and electrically separate the two terminals when the circuit is in “OFF” status. As shown in FIGS. 1 and 2, the safety device includes a bi-metallic plate 201 which has one end fixed to the first terminal 202 and a first contact point 203 connected to the distal end of the bi-metallic plate 201. The second terminal 204 includes a second contact point 205 which is located corresponding to the first contact point 203. When the first and second contact points 203, 205 are in contact with each other, the circuit is in “ON” status and the circuit is in “OFF” status when the first and second contact points 203, 205 are separated.
The safety device is designed to avoid a circuit from being burnt under an overload condition, which generates a high temperature to burn or melt the circuit. The bi-metallic plate 201 is made of two different metal materials and is formed to a plate shape that can deform toward an opposite direction under the overload condition. When the circuit is in “ON” status, the be-metallic plate 201 bends toward the second terminal 204 to keep the first and second contact points 203, 205 in contact with each other as shown in FIG. 1. When the circuit is overloaded, the high temperature makes the bi-metallic plate 201 deform toward an opposite direction so that the first contact point 203 is separated from the second contact point 205 to cut off the circuit as shown in FIG. 2. Accordingly, the electrical appliances can avoid from being burnt.
The bi-metallic plate 201 is made of two different metal materials with different expansion coefficients. When the bi-metallic plate 201 is heated, the bi-metallic plate 201 will deform toward the opposite direction to separate the first and second contact points 203, 205. However, there are several shortcomings for the conventional safety device as follows:    (a) The bi-metallic plates 201 do not trip off perfectly as expected on every time deformation. This is because the bi-metallic plates 201 may have different lengths, thicknesses and ingredients of each of the metal materials, so that some of the bi-metallic plates 201 do not trip off as expected at a pre-set temperature.    (b) The bi-metallic plates 201 do not trip off under overheating condition, which fails to achieve the purpose of protection of the switch.    (c) If the bi-metallic plate 201 does not trip off completely and the circuit is still in “ON” status, the circuit remains at the overheating status and may damage the electrical appliances.    (d) If the bi-metallic plate 201 does not completely trip off, the first and second contact points will be in contact with each other again when the temperature of the circuit goes down within a short period of time. The circuit becomes to be connected and cutoff frequently alternatively, so that the current supplied to the electrical appliances is not stable and the electrical appliances are likely to be damaged.